Impact of no-tillage agricultural systems on sediment yield in two large catchments in Southern Brazil

The impact of agriculture on water resources has long been a problem associated with the formation of runoff, the siltation of lakes and reservoirs, and overall depletion of water quality. In Brazil, these problems are mainly related to soil degradation by water erosion. However, studies of catchment-scale erosion are still rare particularly in grain-producing regions which have adopted conservative tillage systems for soil protection. In order to contribute to a better understanding of the impact of conservation agriculture on water resources, this study determined the runoff coefficient and sediment yield for two agricultural catchments. Hydrological and sedimentological monitoring was conducted in two catchments: the Conceicao catchment is characterized by grain production in weathered soils and a gently sloping landscape, while the Guapore catchment is characterized by heterogeneous soils and topography. Both catchments have problems associated with water erosion. The magnitudes of annual runoff coefficients and sediment yield were high, even if compared to similar agricultural regions, including a catchment with widespread adoption of no-tillage. The sediment yield was 140 t km−2 year−1, and the runoff coefficient was 14 % for the Conceicao catchment, while the sediment yield was 270 t km−2 year−1, and the runoff coefficient was 31 % for the Guapore catchment. The results indicate that problems such as gullies, soil compaction, runoff, floods, siltation, and water quality depletion associated with the misuse of agricultural areas in terms of soil conservation and water use are still evident and important even in regions with widespread adoption of no-tillage systems. The magnitudes of both runoff and sediment yield clearly indicate the need to adopt complementary practices of soil conservation measures, such as mechanical runoff control.